Transmission of digitized voice in packet form is becoming widespread. An example thereof are asychronous transfer mode (ATM) transmission systems. To conserve transmission bandwidth, various techniques have been developed that minimize the amount of information that is sent over the transmission channel while maintaining the perceived quality of the reconstructed speech at the receiving end. Fundamental among these techniques is silence suppression, which is a technique that detects silence intervals in the voice stream at the sending end and suppresses the sending of data representing the silence. Illustrative of these techniques is Time-Assigned Speech Interpolation (TASI), which is a speech-compression technique that examines the digitized voice signal before transmission and eliminates the data which represent silence, so that only data which represents actual speech sounds are transmitted. "Silence" in this context means the absence of speech, and includes the presence of sound below some threshold, e.g., background noise.
A problem with this and other techniques which suppress the transmission of silence is that the recipients of the transmitted and reconstructed voice perceive the lack of the background noise which normally accompanies speech during the silence intervals and rate the quality of the transmission as lower than one which does transmit silence. In an attempt to improve the quality of suppressed-silence transmission systems, synthesized "comfort noise" is added by some systems to the reconstituted voice during silence intervals. Although some improvement in quality is achieved thereby, it does not substantially improve the overall perceived quality of the transmission because the "comfort noise" does not model the actual background noise at the speech source.